Put a space before and after the MBS and the M.B.S. That should do it.
A GREP search would be case-sensitive...
So would a text search if you click the little case sensitive button.
Activate the "Case Sensitive" and the "Whole Word" options in the Find/Change dialog box. The respective buttons are indicated in the screenshot below:
In the screenshot, both buttons are off (it's the default setting).
Remember to deactivate them after you perform this specific search (unless, of course, you want that all all your searches from now on have these settings).